Environmental factors controlling seasonal and spatial variability of zooplankton in thermokarst lakes along a permafrost gradient of Western Siberia.

Humic thermokarst lakes of permafrost peatlands in Western Siberia Lowland (WSL) are major environmental controllers of carbon and nutrient storage in inland waters and greenhouse gases emissions to the atmosphere in the subarctic. In contrast to sizable former research devoted to hydrochemical and hydrobiological (phytoplankton) composition, zooplankton communities of these thermokarst lakes and thaw ponds remain poorly understood, especially along the latitudinal gradient, which is a perfect predictor of permafrost zones. To fill this gap, 69 thermokarst lakes of the WSL were sampled using unprecedented spatial coverage, from continuous to sporadic permafrost zone, in order to assess zooplankton (Cladocera, Copepoda, Rotifera) diversity and abundance across three main open water physiological seasons (spring, summer and autumn). We aimed at assessing the relationship of environmental factors (water column hydrochemistry, nutrients, and phytoplankton parameters) with the abundance and diversity of zooplankton. A total of 74 zooplankton species and taxa were detected, with an average eight taxa per lake/pond. Species richness increased towards the north and reached the maximum in the continuous permafrost zone with 13 species found in this zone only. In contrast, the number of species per waterbody decreased towards the north, which was mainly associated with a decrease in the number of cladocerans. Abundance and diversity of specific zooplankton groups strongly varied across the seasons and permafrost zones. Among the main environmental controllers, Redundancy Analysis revealed that water temperature, lake area, depth, pH, Dissolved Inorganic and Organic Carbon and CO2 concentrations were closely related to zooplankton abundance. Cladocerans were positively related to water temperature during all seasons. Copepods were positively related to depth and lake water pH in all seasons. Rotifers were related to different factors in each season, but were most strongly associated with DOC, depth, CH4, phytoplankton and cladoceran abundance. Under climate warming scenario, considering water temperature increase and permafrost boundary shift northward, one can expect an increase in the diversity and abundance of cladocerans towards the north which can lead to partial disappearance of copepods, especially rare calanoid species.

Contrasted redox-dependent structural control on Fe isotope fractionation during its adsorption onto and assimilation by heterotrophic soil bacteria.

Despite the importance of structural control on metal stable isotope fractionation in inorganic and abiotic systems, the link between metal structural changes and related isotopic fractionation during reactions with organic surfaces and live cells remains poorly established. We conducted reversible adsorption of Fe(II) and Fe(III) on the surface of exopolysaccharide (EPS)-rich and EPS-poor Pseudomonas aureofaciens, and we allowed Fe intracellular uptake by growing cells. We analyzed the Fe isotopic composition of the remaining fluid and cell biomass, and compared the isotopic fractionation during adsorption and assimilation reaction with relative changes in Fe structural status between aqueous solution and bacterial cells, based on available and newly collected X-ray absorption spectroscopy (XAS) observations. Iron(III) adsorption onto P. aureofaciens at 2.8 ≤ pH ≤ 6.0 produced an enrichment of the cell surface in heavier isotopes with Δ57Fecell-solution ranging from +0.7 to +2.1‰, without a link to pH in EPS-rich cultures. In contrast, the magnitude of isotopic fractionation increased with pH in EPS-poor cultures. Iron(II) adsorption produced an even larger enrichment of the cell surface in heavier isotopes, by up to 3.2‰, tentatively linked to Fe(III) hydroxide precipitation. Intracellular assimilation of Fe(II) favored heavier isotopes and led to Δ57Fecell-solution of +0.8‰. In addition, Fe(III) cellular uptake produced an enrichment of the bacterial biomass in lighter isotopes with Δ57Fecell-solution of -1‰. The XAS analyses demonstrated the dominance of Fe(III)-phosphate complexes both at the cell surface and in the cell interior. We suggest that heavier isotope enrichment of the cell surface relative to the aqueous solution is due to strong Fe(III)-phosphoryl surface complexes and Fe complexation to ligands responsible for metal transfer from the surface to the inner cell. In case of Fe(II) adsorption or assimilation, its partial oxidation within the cell compartments may lead to cell enrichment in heavier isotopes. In contrast, loss of symmetry of assimilated Fe(III) relative to the aqueous Fe3+ ion and longer bonds of intracellular ions relative to aqueous Fe(III)-citrate or hydroxo-complexes could produce an enrichment of cells in lighter isotopes. The versatile nature of Fe(II) and Fe(III) fractionation without a distinct effect of pH and surface exopolysaccharide coverage suggests that, in natural soil and sedimentary environments, Fe isotope fractionation during interaction with heterotrophic bacteria will be primarily governed by Fe complexation with DOM and Fe redox status in the soil pore water.

Trace elements in the water column of high-altitude Pyrenean lakes: Impact of local weathering and long-range atmospheric input.

High altitude (alpine) lakes are efficient sentinels of environmental processes, including local pollution and long-range atmospheric transfer, because these lakes are highly vulnerable to ongoing climate changes and increasing anthropogenic pressure. Towards improving the knowledge of trace element geochemistry in the water column of alpine lakes, we assessed 64 physico-chemical parameters, including macro- and micronutrients, major and trace element concentrations in the water column of 18 lakes in the Pyrenees, located along the border between France and Spain. Lake depth, morphology, retention time and watershed rock lithology did not exhibit sizable impact on major and trace element concentrations in the water column. However, acidic (pH = 4.7 ± 0.2) lakes were distinctly different from circumneutral lakes (pH = 6.8 ± 0.5) as they exhibited >10 times higher concentrations of SO42- and trace metals (Fe, Mn, Zn, Cd, Pb, Co, Ni, Be, Al, Ga and REEs). While some of these elements clearly mark the presence of sulphide-rich minerals within the watershed (Fe, Zn, Cd and Pb), the increased mobility of lithogenic elements (Be, Al, Ga and REEs) in acidic lakes may reflect the leaching of these elements from silicate dust derived from atmospheric deposits or surrounding granites. At the same time, compared to circumneutral lakes, acidic lake water displayed lower concentrations of dissolved oxyanions (As, Mo, V, B and W) and elevated SO42- concentrations. The latter could lead to efficient Ba removal from the water column. The exploitation of metal ores within the watershed of three lakes clearly impacted high Zn and Cd concentrations observed in their water column, despite two of these lakes not being acidic. We conclude that local impacts have a greater effect on the water column than long-range atmospheric inputs and that dissolved trace element concentration measurements can be used for revealing sulphide-rich minerals or acid mine drainage within the lakes' watershed.

Laboratory growth capacity of an invasive cyanobacterium (Microcystis aeruginosa) on organic substrates from surface waters of permafrost peatlands.

Within a global warming trend, invasive cyanobacteria, abundant in tropical and temperate regions, can migrate northward and colonize thermokarst lakes in permafrost-affected territories. For a better understanding of the cyanobacterial proliferation mechanism in those lakes, we performed laboratory growth of typical invasive cyanobacteria, Microcystis aeruginosa, onto various organic-rich solutions representative of permafrost peatlands. Aqueous leachates of lichen, moss and peat were the most favorable substrates for massive growth. The growth in the presence of all organic substrates produced an increase in solution pH by two units and a sizable (30-50%) decrease in the concentration of dissolved organic carbon. The observed increase in the dissolved organic carbon aromaticity degree likely reflected preferential cyanobacterial uptake of aliphatic, optically transparent organic substances. Cyanobacterial growth over a bloom period can create a carbon sink (uptake of 2.5 and 8.3 g C-CO2 m-2 d-1) that can offset the net heterotrophic status of thermokarst lakes in permafrost peatlands, thus switching the lake status from a C source to a C sink. Therefore, predictions of future carbon exchanges with the atmosphere in surface waters of permafrost peatlands require explicit accounting for the possibility of invasive cyanobacterial growth.

Fractionation of organic C, nutrients, metals and bacteria in peat porewater and ice after freezing and thawing.

To better understand freezing - thawing cycles operating in peat soils of permafrost landscapes, we experimentally modelled bi-directional freezing and thawing of peat collected from a discontinuous permafrost zone in western Siberia. We measured translocation of microorganisms and changes in porewater chemistry (pH, UV absorbance, dissolved organic carbon (DOC), and major and trace element concentrations) after thawing and two-way freezing of the three sections of 90-cm-long peat core. We demonstrate that bi-directional freezing and thawing of a peat core is capable of strongly modifying the vertical pattern of bacteria, DOC, nutrients, and trace element concentrations. Sizeable enrichment (a factor of 2 to 5) of DOC, macro- (P, K, Ca) and micro-nutrients (Ni, Mn, Co, Rb, B), and some low-mobile trace elements in several horizons of ice and peat porewater after freeze/thaw experiment may stem from physical disintegration of peat particles, leaching of peat constituents, and opening of isolated (non-connected) pores during freezing front migration. However, due to the appearance of multiple maxima of element concentration after a freeze-thaw event, the use of peat ice chemical composition as environmental archive for paleo-reconstructions is unwarranted.

Environmental controllers for carbon emission and concentration patterns in Siberian rivers during different seasons.

Despite the importance of small and medium size rivers of Siberian boreal zone in greenhouse gases (GHG) emission, major knowledge gaps exist regarding its temporal variability and controlling mechanisms. Here we sampled 11 pristine rivers of the southern taiga biome (western Siberia Lowland, WSL), ranging in watershed area from 0.8 to 119,000 km2, to reveal temporal pattern and examine main environmental controllers of GHG emissions from the river water surfaces. Floating chamber measurements demonstrated that CO2 emissions from water surface decreased by 2 to 4-folds from spring to summer and autumn, were independent of the size of the watershed and stream order and did not exhibit sizable (>30 %, regardless of season) variations between day and night. The CH4 concentrations and fluxes increased in the order "spring ≤ summer < autumn" and ranged from 1 to 15 µmol L-1 and 5 to 100 mmol m-2 d-1, respectively. The CO2 concentrations and fluxes (range from 100 to 400 µmol L-1 and 1 to 4 g C m-2 d-1, respectively) were positively correlated with dissolved and particulate organic carbon, total nitrogen and bacterial number of the water column. The CH4 concentrations and fluxes were positively correlated with phosphate and ammonia concentrations. Of the landscape parameters, positive correlations were detected between riparian vegetation biomass and CO2 and CH4 concentrations. Over the six-month open-water period, areal emissions of C (>99.5 % CO2; <0.5 % CH4) from the watersheds of 11 rivers were equal to the total downstream C export in this part of the WSL. Based on correlations between environmental controllers (watershed land cover and the water column parameters), we hypothesize that the fluxes are largely driven by riverine mineralization of terrestrial dissolved and particulate OC, coupled with respiration at the river bottom and riparian sediments. It follows that, under climate warming scenario, most significant changes in GHG regimes of western Siberian rivers located in permafrost-free zone may occur due to changes in the riparian zone vegetation and water coverage of the floodplains.

Kinetics and mechanisms of cyanobacterially induced precipitation of magnesium silicate.

The biomineralization of CO2 , in the form of carbonate minerals, is considered as one of the efficient solutions of atmospheric CO2 removal, allowing stable and sustainable storage of this greenhouse gas. Cyanobacteria are among the most powerful microorganisms capable of precipitating carbonate minerals, both in the present and in the past. In the modern environments, high Si concentration during geoengineering biomineralization could occur due to dissolution of Mg-bearing primary silicates such as olivine. However, most of experimental studies aimed to understand the formation of these carbonates were performed in Si-poor solutions. Thus, experimental characterizations of the nature, rate, and stoichiometry of precipitated minerals in Si-rich solutions in the presence of bacteria are lacking. The present study attempted to reproduce, in controlled laboratory experiments, the processes of biomineralization in a carbonate- and Mg-bearing medium having high Si concentrations (2-4 mM, which is below the saturation with respect to amorphous silica). These experiments have been carried out in the presence of three contrasting cyanobacteria: Synechococcus sp., Chroococcidiopsis sp. and Aphanothece clathrata in order to characterize the rate of formation, stoichiometry and mineralogical nature of precipitates. The results demonstrated significant role of cyanobacteria in the precipitation of carbonate and silicate minerals by increasing the pH of the medium during photosynthesis. Magnesium precipitation rates measured between 50 and 150 h of reaction time ranged from 0.05 to 0.5 mmol h-1 gdry1 and decreased (Synechococcus sp. and Chroococcidiopsis sp.) or increased (A. clathrata) with an increase in the Si:Mg ratio in solution. The abiotic instantaneous rates of Mg and Si removal from alkaline solutions were similar to those in the presence of cyanobacteria at the same pH value suggesting that photosynthetically induced pH rise was the main factor of mineral formation. The transmission electron microscopy (TEM) and spectroscopic observations and associated analyses identified an amorphous magnesium silicate together with hydrous Mg carbonates (hydromagnesite). The formation of carbonate solid phase at high Mg: Si ratios indicated the potential for the removal of inorganic carbon at pH > 10. The difference in the degree of C removal between different species was primarily linked to different degree of pH rise during photosynthesis. Taken together, the results obtained in this study allowed an efficient reproduction of combined magnesium hydroxo-carbonates and hydrous silicates precipitation under cyanobacterial activity, suitable for geoengineering of biologically controlled CO2 sequestration in Si-Mg-carbonate-bearing solutions.

Organic carbon, and major and trace elements reside in labile low-molecular form in the ground ice of permafrost peatlands: a case study of colloids in peat ice of Western Siberia.

The fate of organic carbon (OC), nutrients and metals accumulated in thawing permafrost ice is at the forefront of environmental studies in the Arctic. In contrast to a fairly good understanding of the chemical nature of dissolved OC (DOC) and metals in surface Arctic waters, the speciation and colloidal status of solutes accommodated in the dispersed ground ice remain virtually unknown. Here we used a size fractionation procedure (centrifugal ultrafiltration) to quantify the proportion of colloidal (3 kDa to 0.45 µm) and conventionally dissolved low molecular weight (LMW<3 kDa) fractions of DOC, and major and trace elements in the porewater and ice of 5 peat cores sampled along a 400 km permafrost and climate gradient in the largest peatland in the world, the Western Siberian Lowland (WSL). We discovered that the strong (a factor of 2 to 10) increase in the total dissolved (<0.45 µm) concentration of DOC and most major and trace elements in the peat ice relative to the peat porewater from the thawed layer was essentially linked to an increase in the LMW<3 kDa fraction. This increase in the potentially bioavailable fraction in the peat ice relative to the porewater was especially pronounced for DOC, P and many trace elements including metal micronutrients, and was observed throughout all permafrost zones. This contrasted with element distribution in the upper (thaw) layer, where the majority of these elements were present in the colloidal pool. Following previous experiments on permafrost peatland surface waters, we hypothesized that the freeze-thaw cycles of peat porewater were responsible for generation of the LMW fraction in the bottom part of the peat core. Results of this study demonstrate that carbon, and macro- and micro-nutrients as well as trace metals in ground ice of permafrost peatlands are essentially present in a low molecular weight (<3 kDa) and potentially bioavailable form that can strongly impact the riverine export fluxes of solutes during permafrost thaw.

Carbon, nutrient and metal controls on phytoplankton concentration and biodiversity in thermokarst lakes of latitudinal gradient from isolated to continuous permafrost.

Shallow thaw (thermokarst) lakes abundant in regions of permafrost-affected peatlands represent important sources of carbon dioxide and methane emission to the atmosphere, however the quantitative parameters of phytoplankton communities which control the C cycle in these lakes remain poorly known. This is especially true considering the roles of permafrost, hydrochemical composition of lakes, lake sizes and season as major governing factors on phytoplankton abundance and biodiversity. In this work, we quantified phytoplankton characteristics of 27 thermokarst lakes (sizes ranging from 115 m2 to 1.24 km2) sampled in spring, summer and autumn across a permafrost gradient (isolated, sporadic, discontinuous and continuous zone) in the Western Siberia Lowland (WSL). The biodiversity indices were highest during all seasons in lakes of the continuous permafrost zone and rather similar in lakes of isolated, sporadic and discontinuous permafrost zone. Considering all seasons and permafrost zones, the biomass and cell number of phytoplankton correlated with Dissolved Organic Carbon (DOC), phosphate, and some metal micro-nutrients (Ni, Zn). The strongest correlations were observed for Cyanophycea during summer, with pH, Ni, Cu, Zn, Sr, Ba (cell number) and Cu, Zn, Ba (biomass), and during autumn, with DOC, K, Cr, Cu, Zn, Ba, Cd, Pb (biomass). Using a substituting space for time approach for climate warming and permafrost thaw and suggesting a shift in permafrost boundaries northward, we foresee an increase in cell number and biomass in continuous permafrost zone in spring and summer, and a decrease in phytoplankton abundance in the discontinuous and sporadic permafrost zones. The biodiversity of phytoplankton in the continuous permafrost zone might decrease whereas in other zones, it may not exhibit any sizably change. However, in case of strong deepening of the active layer down to underlaying mineral horizons, and the release of some limiting nutrients (Si, P) due to enhanced connectivity of the lake with groundwater, the share of cyanobacteria and diatoms may increase.

Impact of freeze-thaw cycles on organic carbon and metals in waters of permafrost peatlands.

Despite the importance of soil and surface waters freezing in permafrost landscapes, the behaviour of dissolved organic carbon (DOC), nutrients and metals during periodic freeze-thaw cycles (FTC) remains poorly known. The on-going climate warming is likely to increase the frequency of FTC in continental aquatic settings, which could modify the chemical composition of waters. In this study, we conducted 9 repetitive cycles of overnight freezing (-20 °C) and 5 h thawing (4 °C) in the laboratory using representative 0.22 µm-filtered waters from NE European permafrost peatland: leachates of vegetation and soil, and natural surface waters (depression, thermokarst lake and river). Only minor (<5%-15%) changes of DOC concentrations, SUVA254 and molecular weight were observed in all leachates and the depression water. In contrast, several trace elements (Fe, Al, P, Mn, As, and REE) exhibited sizable variations during FTC (>10%). The leachates and the depression water were enriched in trace elements, whereas the thermokarst lake and the river demonstrated a decrease in concentration of Fe (-39 and -94%, respectively), Al (-9 and -85%), and Mn (-10 and -79%) during FTC. Overall, the observations demonstrated an increase in aliphatic low molecular weight organic matter (OM), and the precipitation of Fe, Al hydroxides and organo-mineral particles. Therefore, enhanced of frequency of FTC can favour the release of metals and toxicants from acidic OM-rich surface waters and maintain stable OM-metals-colloids in large lakes and rivers, thus regulating aquatic transport of DOC and metals from soils to the Arctic Ocean.

Lichen, moss and peat control of C, nutrient and trace metal regime in lakes of permafrost peatlands.

Permafrost thaw in continental lowlands produces large number of thermokarst (thaw) lakes, which act as a major regulator of carbon (C) storage in sediments and C emission in the atmosphere. Here we studied thaw lakes of the NE European permafrost peatlands - shallow water bodies located within frozen peat bogs and receiving the majority of their water input from lateral (surface) runoff. We also conducted mesocosm experiments via interacting lake waters with frozen peat and dominant ground vegetation - lichen and moss. There was a systematic decrease in concentrations of dissolved C, CO2, nutrients and metals with an increase in lake size, corresponding to temporal evolution of the water body and thermokarst development. We hypothesized that ground vegetation and frozen peat provide the majority of C, nutrients and inorganic solutes in the water column of these lakes, and that microbial processing of terrestrial organic matter controls the pattern of CO2 and nutrient concentrations in thermokarst lakes. Substrate mass-normalized C, nutrient (N, P, K), major and trace metal release was maximal in moss mesocosms. After first 16 h of reaction, the pCO2 increased ten-fold in mesocosms with moss and lichen; this increase was much less pronounced in experiments with permafrost peat. Overall, moss and lichen were the dominant factors controlling the enrichment of the lake water in organic C, nutrients, and trace metals and rising the CO2 concentration. The global significance of obtained results is that the changes in ground vegetation, rather than mere frozen peat thawing, may exert the primary control on C, major and trace element balance in aquatic ecosystems of tundra peatlands under climate warming scenario.

Biogeochemistry of macrophytes, sediments and porewaters in thermokarst lakes of permafrost peatlands, western Siberia.

The chemical composition of thermokarst lake ecosystem components is a crucial indicator of current climate change and permafrost thaw. Despite high importance of macrophytes in shallow permafrost thaw lakes for control of major and trace nutrients in lake water, the trace element (TE) partitioning between macrophytes and lake water and sediments in the permafrost regions remains virtually unknown. Here we sampled dominant macrophytes in thermokarst lakes of discontinuous and continuous permafrost zones in the Western Siberia Lowland (WSL) and measured major and trace elements in plant biomass, lake water, lake sediments and sediment porewater. All six plant species (Hippuris vulgaris L., Glyceria maxima (Hartm.) Holmb., Comarum palustre L., Ranunculus spitzbergensis Hadac, Carex aquatilis Wahlenb s. str., Menyanthes trifoliata L.) sizably accumulated macronutrients (Na, Mg, Ca), micronutrients (B, Mo, Nu, Cu, Zn, Co) and toxicants (As, Cd). Accumulation of other trace elements, including rare earth elements (REE), in macrophytes relative to pore waters and sediments was highly variable among species. Using miltiparametric statistics, we described the behavior of ТЕ across two permafrost zones and identified several group of elements depending on their sources in the lake ecosystems and their affinity to sediments and macrophytes. Under future climate warming and shifting the permafrost border to the north, we anticipate an increasing uptake of heavy metals and lithogenic low mobile elements such as Ti, Al, Cr, As, Cu, Fe, Ni, Ga, Zr, and REEs by macrophytes in the discontinuous permafrost zone and Ba, Zn, Pb and Cd in the continuous permafrost zone. This may eventually diminish transport of metal micronutrients and geochemical tracers from soils to lakes and rivers and further to the Arctic Ocean.

Dispersed ground ice of permafrost peatlands: Potential unaccounted carbon, nutrient and metal sources.

The physical and chemical consequences of massive ground ice (wedges) melt upon permafrost thaw is one of the central issues of environmental research linked to climate warming in the Arctic. Little is known about the chemical properties of dispersed ground ice abundant throughout permafrost peatlands that can easily melt with increasing active layer thickness (ALT). This is especially pertinent in continental lowlands, that account for sizeable areas of the Arctic, and contain high amount of organic carbon in both solid (peat) and liquid (porewater) phases. Here we studied 8 peat cores (0-130 cm depth)-comprised of porewater from the active layer (0-45 cm) as well as ice dispersed in frozen peat (40-130 cm)-across a latitudinal profile of Western Siberia Lowland (WSL) extending from discontinuous into continuous permafrost zones. Dissolved Organic Carbon (DOC), alkali and alkaline-earth metals (Ca, Mg, Sr, Ba, Li, Rb, Cs), sulfate, phosphorus, some trace elements (Al, Fe, Mn, Zn, Ni, Co, V, As, Y, REE, Zr, Hf, U) were sizably [more than 3 times] enriched in peat ice compared to peat porewaters from the active layer. In most sampled cores, there was a local maximum of strong enrichment (up to factors between 14 and 58) in DOC, P, Ca, Mg, Mn, Fe, Sr, As located 30-50 cm below the active layer. This maximum likely occurred due to solute concentration during full freezing of the soil column during winter. There was a sizable correlation between DOC, Al, Fe and other major and trace element concentrations that suggests strong control of organic complexes and organo-mineral (Al, Fe) colloids on element migration throughout the peat profile. The pool of C, major cations and trace metals in peat ice (40-130 cm) was approximately 3-55 times higher than the pool of these elements in porewaters from the active layer (0-40 cm). A 1-m increase of the ALT over the next 100 years is capable of mobilizing 58 ± 38 Tg of DOC from soil ice into the rivers and lakes of the WSL latitudinal belt (63-67 °N). This fast lateral export of C (3.7 ± 2.7 t C km-2 y-1) may double current C yields in WSL rivers (3.4 ± 1.3 t C km-2 y-1). A strong increase (150-200%) in riverine export of Zn, P and Cs may also occur while other micronutrients (Fe, Ni, Co, Ba, Mo, Rb) and toxicants (Cd, As, Al) may be affected to a lesser degree (20-30% increase). We propose a global peat ice inventory in permafrost regions is essential for assessing the consequences of permafrost thaw on surface aquatic systems.

Iron Isotope Fractionation during Bio- and Photodegradation of Organoferric Colloids in Boreal Humic Waters.

Biodegradation and photolysis of dissolved organic matter (DOM) in boreal high-latitude waters are the two main factors controlling not only the aquatic fluxes and residence time of carbon but also metal nutrients associated with DOM such as Fe. The DOM is usually present in the form of organic and organomineral colloids, which also account for the majority of dissolved Fe. Here, we use the stable Fe isotope approach to unravel the processes controlling Fe behavior during bio- and photodegradation of colloids in boreal Fe- and DOM-rich humic waters (a stream and a fen). The adsorption of Fe colloids onto heterotrophic bacteria Pseudomonas aureofaciens produced enrichment in +0.4‰ (δ57Fe) in the heavier isotopes of the cell surface relative to the remaining solution. In contrast, long-term assimilation of Fe by live cells yielded preferential incorporation of lighter isotopes into the cells (-0.7‰ relative to aqueous solution). The sunlight-induced oxidation of Fe(II) in fen water led to the removal of heavier Fe isotopes (+1.5 to +2.5‰) from solution, consistent with Fe(III) hydroxide precipitation from Fe(II)-bearing solution. Altogether, bio- and photodegradation of organoferric colloids, occurring within a few days of exposure time, can produce several per mil isotopic excursions in shallow lentic and lothic inland waters of high-latitude boreal regions. Considerable daily scale variations of Fe isotopic composition should therefore be taken into account during the interpretation of the riverine flux of Fe isotopes to the ocean or tracing weathering processes using Fe isotopes in surface waters at high latitudes.

Dissolved Organic Matter Controls Seasonal and Spatial Selenium Concentration Variability in Thaw Lakes across a Permafrost Gradient.

Little is known about the sources and processing of selenium, an important toxicant and essential micronutrient, within boreal and sub-arctic environments. Upon climate warming and permafrost thaw, the behavior of Se in northern peatlands becomes an issue of major concern, because a sizable amount of Se can be emitted to the atmosphere from thawing soils and inland water surfaces and exported to downstream waters, thus impacting the Arctic biota. Working toward providing a first-order assessment of spatial and temporal variation of Se concentration in thermokarst waters of the largest frozen peatland in the world, we sampled thaw lakes and rivers across a 750-km latitudinal profile. This profile covered sporadic, discontinuous, and continuous permafrost regions of western Siberia Lowland (WSL), where we measured dissolved (<0.45 µm) Se concentration during spring (June), summer (August), and autumn (September). We found maximum Se concentration in the discontinuous permafrost zone. Considering all sampled lakes, Se exhibited linear relationship ( R2 = 0.7 to 0.9, p < 0.05, n ≈ 70) with dissolved organic carbon (DOC) concentration during summer and autumn. Across the permafrost gradient, the lakes in discontinuous permafrost regions demonstrated stronger relationship with DOC and UV-absorbance compared to lakes in sporadic/isolated and continuous permafrost zones. Both seasonal and spatial features of Se distribution in thermokarst lakes and ponds suggest that Se is mainly released during thawing of frozen peat. Mobilization and immobilization of Se within peat-lake-river watersheds likely occurs as organic and organo-Fe, Al colloids, probably associated with reduced and elemental Se forms. The increase of active layer thickness may enhance leaching of Se in the form of organic complexes with aromatic carbon from the deep horizons of the peat profile. Further, the northward shift of permafrost boundaries in WSL may sizably increase Se concentration in lakes of continuous permafrost zone.

Photodegradation of river dissolved organic matter and trace metals in the largest European Arctic estuary.

Photo-induced degradation of dissolved organic matter (DOM) and organo-mineral colloids is one of the major factor responsible for transformation of DOM and dissolved metals in boreal and subarctic waters. In contrast to fairly good understanding of this process in inland waters of high latitude zone, the transformation of riverine DOM and associated trace element (TE) colloids in the Arctic estuaries remains virtually unknown. We incubated, under sunlight in outdoor pools, quartz reactors filled with mixtures of sterile filtered riverine and estuarine water. The water samples were collected in the estuarine zone of the largest European Arctic river, Severnaya Dvina. After 1month of exposure to sunlight, ≤5% change of dissolved organic carbon (DOC) concentration and specific ultraviolet (254nm) absorption occurred. This decrease was within the experimental uncertainty and it implies quite high resistance of river dissolved organic matter to photo-degradation in this estuary. Moreover, very low photodegradability of DOM in the freshwater point of the Severnaya Dvina River may require revisiting the current paradigm of the importance of DOC photolysis in large Arctic rivers. A novel finding was that the percentages of overall removal of Fe and some insoluble elements were quite similar across the full range of studied salinities, whereas the apparent rate of metal removal decreased with the increase of salinity. Overall, the salinity weakly impacted the removal of riverine DOC and metals in the estuarine water via photolysis and coagulation under sunlight. As a result, photoreactivity of DOM and dissolved metals in riverine end members corrected for estuarine dilution can be used to approximate the photolytic transformation of riverine material in the Arctic coastal zone.

Low biodegradability of dissolved organic matter and trace metals from subarctic waters.

The heterotrophic mineralization of dissolved organic matter (DOM) controls the CO2 flux from the inland waters to the atmosphere, especially in the boreal waters, although the mechanisms of this process and the fate of trace metals associated with DOM remain poorly understood. We studied the interaction of culturable aquatic (Pseudomonas saponiphila) and soil (Pseudomonas aureofaciens) Gammaproteobacteria with seven different organic substrates collected in subarctic settings. These included peat leachate, pine crown throughfall, fen, humic lake, stream, river, and oligotrophic lake with variable dissolved organic carbon (DOC) concentrations (from 4 to 60mgL-1). The highest removal of DOC over 4days of reaction was observed in the presence of P. aureofaciens (33±5%, 43±3% and 53±7% of the initial amount in fen water, humic lake and stream, respectively). P. saponiphila degraded only 5% of DOC in fen water but did not affect all other substrates. Trace elements (TE) were essentially controlled by short-term (0-1h) adsorption on the surface of cells. Regardless of the nature of organic substrate and the identity of bacteria, the degree of adsorption ranged from 20 to 60% for iron (Fe3+), 15 to 55% for aluminum (Al), 10 to 60% for manganese (Mn), 10 to 70% for nickel (Ni), 20 to 70% for copper (Cu), 10 to 60% for yttrium (Y), 30 to 80% for rare earth elements (REE), and 15 to 50% for uranium (UVI). Rapid adsorption of organic and organo-mineral colloids on bacterial cell surfaces is novel and potentially important process, which deserves special investigation. The long-term removal of dissolved Fe and Al was generally consistent with solution supersaturation degree with respect to Fe and Al hydroxides, calculated by visual Minteq model. Overall, the biomass-normalized biodegradability of various allochthonous substrates by culturable bacteria is much lower than that of boreal DOM by natural microbial consortia.

Experimental modeling of thaw lake water evolution in discontinuous permafrost zone: Role of peat, lichen leaching and ground fire.

Thaw of frozen peat in discontinuous permafrost zone produces a significant number of thermokarst lakes, which are known to contribute to Green House Gases (GHG) emission in the atmosphere. In palsa peatland of western Siberia, the thermokarst lake formation includes soil subsidences, lichen submergence and peat abrasion, leading to lateral spreading of the lake border, often intensified by ground fires. Mesocosm experiments were conducted during 3weeks on two thermokarst lake waters interacting in 30-L tanks with surface horizon of peat, the dominant ground vegetation (lichen Cladonia sp.) and the ash produced by lichen burning at 450°C. The obtained results allowed a better understanding of physico-chemical factors controlling the enrichment of thermokarst lake water in organic carbon and metals, and evaluating CO2 sequestration/emission potential. The changes of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC), major element and divalent metal concentration in response to peat and lichen biomass addition were less than a factor of 2 over full duration of the experiment. Iron (Fe) concentration in the lake water decreased by a factor of 2 to 3 after the addition of peat and lichen biomass. The concentration of low-soluble trivalent and tetravalent hydrolysates decreased by ca. 30 to 50%, presumably due to their co-precipitation with Fe hydroxide. The dissolved carbon dioxide (CO2) in tank with lichen increased by a factor of 5.5±0.5, likely due to respiration of algal component in closed environment. Strong enrichment of the lake water in DIC, P, K, Ca, Mg, Si, Al, Ti, Mn, Mo, Rb, As, Sb and U upon the ash addition persisted over full duration of experiments and was significant (p<0.0001) compared to peat and lichen biomass treatments. These elements may serve as indicators of ground fire impact on thermokarst lake water's chemistry. The overall effect of ash leaching on aquatic ecosystems after ground fire of frozen Siberian peatland is predicted to be much stronger than that currently recognized for non-permafrost regions.

Zeta potential of anoxygenic phototrophic bacteria and Ca adsorption at the cell surface: possible implications for cell protection from CaCO3 precipitation in alkaline solutions.

Electrophoretic mobility measurements and surface adsorption of Ca on living, inactivated, and heat-killed haloalkaliphilic Rhodovulum steppense, A-20s, and halophilic Rhodovulum sp., S-17-65 anoxygenic phototrophic bacteria (APB) cell surfaces were performed to determine the degree to which these bacteria metabolically control their surface potential equilibria. Zeta potential of both species was measured as a function of pH and ionic strength, calcium and bicarbonate concentrations. For both live APB in 0.1M NaCl, the zeta potential is close to zero at pH from 2.5 to 3 and decreases to -30 to -40 mV at pH of 5-8. In alkaline solutions, there is an unusual increase of zeta potential with a maximum value of -10 to -20 mV at a pH of 9-10.5. This increase of zeta potential in alkaline solutions is reduced by the presence of NaHCO(3) (up to 10 mM) and only slightly affected by the addition of equivalent amount of Ca. At the same time, for inactivated (exposure to NaN(3), a metabolic inhibitor) and heat-killed bacteria cells, the zeta potential was found to be stable (-30 to -60 mV, depending upon the ionic strength) between pH 5 and 11 without any increase in alkaline solutions. Adsorption of Ca ions on A-20s cells surface was more significant than that on S-17-65 cells and started at more acidic pHs, consistent with zeta potential measurements in the presence of 0.001-0.01 mol/L CaCl(2). Overall, these results indicate that APB can metabolically control their surface potential to electrostatically attract nutrients at alkaline pH, while rejecting/avoiding Ca ions to prevent CaCO(3) precipitation in the vicinity of cell surface and thus, cell incrustation.